Steel for induction hardening, roughly shaped material for induction hardening, producing method thereof, and induction hardening steel part

ABSTRACT

A steel for an induction hardening including, by mass %, C: more than 0.75% to 1.20%, Si: 0.002 to 3.00%, Mn: 0.20 to 2.00%, S: 0.002 to 0.100%, Al: more than 0.050% to 3.00%, P: limited to 0.050% or less, N: limited to 0.0200% or less, O: limited to: 0.0030% or less, and the balance composing of iron and unavoidable impurities, wherein an Al content and a N content satisfy, by mass %, Al−(27/14)×N&gt;0.050%.

TECHNICAL FIELD

The present invention relates to a steel for induction hardening, aroughly shaped material for induction hardening, a producing methodthereof, and an induction hardening steel part.

This application is a national stage application of InternationalApplication No. PCT/JP2011/053109, filed Feb. 15, 2011, which claimspriority to Japanese Patent Application No. 2010-078232, filed Mar. 30,2010, the content of which is incorporated herein by reference.

BACKGROUND ART

Most power transmission parts (for example, gears, bearings, CVTsheaves, shafts, and the like) used for automobiles, constructionmachines, farm machines, electricity generating-wind turbines, otherindustrial machines and the like are used after being subjected to asurface hardening treatment to improve, for example, fatiguecharacteristics and abrasion resistance of the parts. Among a pluralityof known surface hardening treatments, a carburizing treatment issuperior to other surface hardening treatments in terms of the surfacehardness, depth of hardened layer, productivity, and the like, so thecarburizing treatment is applied to numerous types of the parts. Forexample, in a manufacturing method of the gears and bearing parts, byusing medium carbon alloy steel such as SCM 420, SCR 420, SNCM 220 ingeneral, and the like specified by JIS, a mechanical processing isperformed to obtain a predetermined shape through a hot forging, a coldforging, cutting, or through a combination thereof, and then thecarburizing treatment or a carbonitriding treatment is performed. Thefatigue fracture of the gears is classified roughly into bending fatigue(dedendum fatigue) and tooth surface fatigue (pitting or the like). Inorder for the gear parts to obtain durability, both the above types offatigue strength need to be improved. Since the gears which are producedby the carburizing treatment have very high hardness of hardening layer,there is the feature of having the performance excellent in both thebending fatigue strength and the fatigue strength.

However, the carburizing treatment is batch processing in the gasatmosphere. For example, the carburizing treatment requires heating andholding around 930° C. for several hours or more, so that significantoccupancy expense, treatment energy, and cost are needed. Thecarburizing treatment emits a large volume of CO₂, so that there is aproblem in terms of the environment. Since the carburizing treatment isthe batch processing, the carburizing treatment has the problems whichare that the dispersion of part accuracy becomes possibly large becauseof the heat treatment deformation caused by the difference of theloading position of the parts at the carburizing treatment and that theaccuracy control of the parts is difficult. In order to solve theproblems concerned with the heat treatment deformation, huge effort hasbeen made in regard to materials and operations, and then theimprovement effect to a certain extent has been obtained. However, aradical method of settlement is still not found out, and it is said thatthe improvement effect does not reach enough levels.

In order to solve the problems, the research on application of inductionhardening (electromagnetic induction hardening) treatment to substitutethe carburizing treatment has been made. Since the induction hardeningtreatment can reduce considerably the energy and the time for thetreatment compared with the carburizing treatment, the inductionhardening treatment has the advantages of the productivity and the costreduction. Furthermore, the induction hardening treatment does not emita large volume of CO₂ and quenching oil to environment, so that there isthe advantage for the environment. In addition, since the area which isaffected by the influence of the heat treatment is limited to thesurface unlike the carburizing treatment, the heat treatment deformationby the induction hardening treatment is essentially small. Moreover,there are the advantages which are that consecutive processing becomespossible because processing time is short and that the accuracy controlof the parts becomes easy because the dispersion of the heat treatmentdeformation is small.

On the other hand, although there are the above mentioned advantages,the induction hardening treatment has not become common as thesubstitution of the carburizing treatment. The prime reasons thereof arebecause coexistence between the securement of tooth surface fatiguestrength (pitching strength and the like) of the parts and workability(machinability or cold forgeability) during production of the parts isvery difficult. Not only the gears but also CVT sheaves and bearingsneed to improve surface fatigue such as tooth surface fatigue androlling fatigue. It is reported that hardness at 300° C. (or hardnessafter tempering at 300° C., hereinafter referred to as 300° C. temperedhardness) correlates strongly with the surface fatigue strength sincesurface temperature of the contact surface of the parts rises up toabout 300° C. while using the parts. The 300° C. tempered hardness ofmartensite structure obtained by the carburizing treatment or theinduction hardening treatment improves with an increase in carboncontent in surface layer. Although the 300° C. tempered hardness isaffected by addition of alloying elements, the influence of the carboncontent is greater. An improvement effect of the 300° C. temperedhardness by the addition of the alloying elements increases with theincrease in the carbon content. Therefore, in order to obtain thesurface fatigue strength equivalent to carburized parts, it is necessarythat the carbon content (about 0.80%) is equivalent to the carboncontent in the surface layer of the carburized parts. However, theincrease in the carbon content of the parts results in the increase inhardness of base steel, so that the workability (the machinability orthe cold forgeability) of the parts deteriorates remarkably, which isnot suitable for industrial production. It is indispensable to coexistbetween high carbon content of the base steel and securement of theworkability.

For example, Patent Documents 1 to 6 suggest the technique of producingparts by performing the induction hardening to medium carbon steels (C:to 0.65%). However, since the carbon content is considerably less thanthat of the surface layer of the carburized parts, the workability doesnot deteriorate so much, but the tooth surface fatigue strengthdecreases compared with the carburized parts. For this reason, thetechnique cannot be substituted for the carburizing. For example, PatentDocuments 7 to 13 suggest the technique of obtaining the parts in whichthe tooth surface fatigue strength is improved by performing theinduction hardening to comparative high carbon steels (C: to 0.75%).However, since the carbon content is still less than that of the surfacelayer of the carburized parts, the tooth surface fatigue strength whichis equal to that of the carburized parts is not obtained. Moreover, theworkability decreases notably with the increase in the carbon content inthe steels. However, since the improvement technique for this isinsufficient, both the tooth surface fatigue strength and theworkability are insufficient after all, so that the technique cannot besubstituted for the carburizing.

For example, Patent Documents 14 to 17 suggest the technique which is toimprove the workability and the like by providing suitable rollingconditions, forging conditions, and cooling conditions to comparativehigh carbon steels (C: to 0.75%). However, as described above, since thecarbon content is still less than that of the surface layer of thecarburized parts, the tooth surface fatigue strength which is equal tothat of the carburized parts is not obtained, so that the techniquecannot be substituted for the carburizing.

For example, Patent Documents 18 to 23 suggest the technique in which aheat treatment is performed if necessary and then the inductionhardening is performed to the steels which have the high carboncomposition which is equal to that of the surface layer of thecarburized parts. By the technique, a hardening layer with the structurein which alloy carbides are dispersed in the martensite structure isformed, so that the parts which have the high tooth surface fatiguestrength are obtained. However, in the technique, alloy addition such asCr, V, Ti, Nb, and the like is large in order to disperse the alloycarbides. Thus, although the tooth surface fatigue strength which isgreater than that of the carburized parts is obtained, the workabilitydecreases notably by both the increase of the carbon content and theincrease of the alloy addition. Therefore, except for application onsome special parts, since the application and the practical realizationfor mass products are difficult in terms of the cost, the productivity,and the like, it cannot be said that the technique is practical tosubstitute for the carburizing.

For example, Patent Documents 24 to 26 suggest the technique in whichthe heat treatment is performed if necessary and then the inductionhardening is performed to the steels which have the high carboncomposition which is equal to that of the surface layer of thecarburized parts, in order to obtain the parts in which the toothsurface fatigue strength is improved. However, since the improvement forthe workability is insufficient, the technique also cannot besubstituted for the carburizing.

For example, Patent Document 27 suggests the technique which is toimprove the machinability by precipitating graphites to a certain extentby using the high carbon steels (C: 0.80 to 1.50%). Although the exampleof application to the induction hardening steel part is also shown inthe patent documents 27, in the base steel in which a lot of thegraphites are dispersed, it is difficult to solute the graphites assolid solution in matrix, and there is a problem such that voids areformed at the position where the graphites existed. For this reason, inthe method, various characteristics for the power transmission partswhich require reliability deteriorate. In order to perform solution ofthe graphites or dissolution of the voids, the induction hardeningshould be performed by the special conditions which are at a hightemperature and for a long time. For this reason, problems which arethat the control of depth of the hardening layer is impossible or theproductivity deteriorates occur. In this case, the above mentionedadvantageous feature of the induction hardening is not obtained at all.Therefore, it cannot be said that the technique of dispersing a lot ofgraphites is practical to apply to the induction hardening treatment ofthe power transmission parts.

CITATION LIST Patent Document

-   [Patent Document 1] Japanese Unexamined Patent Application, First    Publication No. S62-112727-   [Patent Document 2] Japanese Patent No. 3239432-   [Patent Document 3] Japanese Unexamined Patent Application, First    Publication No. H9-291337-   [Patent Document 4] Japanese Unexamined Patent Application, First    Publication No. 2000-319725-   [Patent Document 5] Japanese Unexamined Patent Application, First    Publication No. H11-269601-   [Patent Document 6] Japanese Unexamined Patent Application, First    Publication No. 2000-144307-   [Patent Document 7] Japanese Unexamined Patent Application, First    Publication No. H7-118791-   [Patent Document 8] Japanese Unexamined Patent Application, First    Publication No. H11-1749-   [Patent Document 9] Japanese Patent No. 3208960-   [Patent Document 10] Japanese Patent No. 3503289-   [Patent Document 11] Japanese Patent No. 3428282-   [Patent Document 12] Japanese Patent No. 3562192-   [Patent Document 13] Japanese Patent No. 3823413-   [Patent Document 14] Japanese Patent No. 3458604-   [Patent Document 15] Japanese Patent No. 3550886-   [Patent Document 16] Japanese Patent No. 3644217-   [Patent Document 17] Japanese Patent No. 3606024-   [Patent Document 18] Japanese Patent No. 3607583-   [Patent Document 19] Japanese Unexamined Patent Application, First    Publication No. 2002-53930-   [Patent Document 20] Japanese Unexamined Patent Application, First    Publication No. 2005-163173-   [Patent Document 21] Japanese Patent No. 4390526-   [Patent Document 22] Japanese Patent No. 4390576-   [Patent Document 23] Japanese Unexamined Patent Application, First    Publication No. 2009-102733-   [Patent Document 24] Japanese Unexamined Patent Application, First    Publication No. H8-73929-   [Patent Document 25] Japanese Unexamined Patent Application, First    Publication No. 2004-300551-   [Patent Document 26] Japanese Unexamined Patent Application, First    Publication No. 2008-248282-   [Patent Document 27] Japanese Unexamined Patent Application, First    Publication No. H11-350066

SUMMARY OF INVENTION Technical Problem

In view of the above-mentioned problems, an object of the invention isto provide a steel for induction hardening, a roughly shaped materialfor the induction hardening, a producing method thereof, and aninduction hardening steel part, in which fatigue strength (tooth surfacefatigue strength, tooth root fatigue strength, and the like) of thesteel part after the induction hardening is equal to or greater thanthat of carburized parts, and coexistence between securement of thefatigue strength of the steel part and workability during production ofthe part is possible.

Solution to Problem

Hereinafter the steel, which is cast in order to produce an inductionhardening steel part and for which soaking, blooming, and the like areconducted if necessary, is referred to as a steel for inductionhardening. The roughly shaped intermediate material, which is made by atleast one process of warm forging, hot forging, hot rolling, slowcooling, annealing, and the like to the steel for the inductionhardening, is referred to as a roughly shaped material for the inductionhardening (or referred to as a base steel, or simply a roughly shapedmaterial). An induction hardening steel part is produced by theinduction hardening and other processes as necessary, following cutting,cold forging, and/or the like to the roughly shaped material.

In order to solve the above-mentioned problems, the inventors haveinvestigated and then found the following results.

-   -   (a) The controlling factor of the strength of the high carbon        base steel with carbon content of greater than 0.75% is the        strength of a pearlite. Thus, by performing an annealing on        suitable conditions when producing the roughly shaped material        before cutting or cold forging, softening can be obtained by a        decrease in the strength of the pearlite, so that machinability        and cold forgeability can be improved.    -   (b) In case that the roughly shaped material is made by hot        working, by performing a suitable cooling in cooling process        after the hot working, softening can be obtained by a decrease        in the strength of the pearlite.    -   (c) By not adding an alloying element excessively but increasing        Al content considerably compared with conventional steel as a        steel composition, a decrease in the machinability can be        suppressed even if the strength of the roughly shaped material        before cutting increases by an increase in the carbon content.

The inventors completed the present invention by utilizing suitably theabove mentioned techniques. An aspect of the present invention employsthe following.

(1) A steel for an induction hardening according to an aspect of theinvention comprising, by mass %,

C: more than 0.75% to 1.20%,

Si: 0.002 to 3.00%,

Mn: 0.20 to 2.00%,

S: 0.002 to 0.100%,

Al: more than 0.050% to 3.00%,

P: limited to 0.050% or less,

N: limited to 0.0200% or less,

O: limited to 0.0030% or less, and

the balance consisting of iron and unavoidable impurities,

wherein an Al content and a N content satisfy, by mass %,Al−(27/14)×N>0.050%.

(2) The steel for the induction hardening according to (1) may furthercomprise, by mass %,

B: 0.0005 to 0.0050%.

(3) The steel for the induction hardening according to (1) or (2) mayfurther comprise at least one of, by mass %,

Cr: 0.05% to less than 0.30%,

Mo: 0.01 to 1.00%,

Cu: 0.05 to 1.00%, and

Ni: 0.05 to 2.00%.

(4) The steel for the induction hardening according to any one of (1) to(3) may further comprise at least one of, by mass %,

V: 0.005 to less than 0.20%,

Nb: 0.005 to 0.10%, and

Ti: 0.005 to 0.10%.

(5) The steel for the induction hardening according to any one of (1) to(4) may further comprise at least one of, by mass %,

Ca: 0.0005 to 0.0030%,

Zr: 0.0005 to 0.0030, and

Mg: 0.0005 to 0.0030%.

(6) A roughly shaped material for the induction hardening which has achemical composition of the steel for the induction hardening accordingto any one of (1) to (5),

wherein a number of graphite grains with an average grain size of 0.5 μmor more which is included in the roughly shaped material for theinduction hardening is 40 pieces/mm² or less.

(7) A producing method of a roughly shaped material for the inductionhardening,

wherein processes of warm working or hot working, cooling, and annealingare performed in order by using the steel for the induction hardeningaccording to any one of (1) to (5), and

wherein the annealing is performed by a condition of an annealingtemperature of 680 to 800° C. and an annealing time of 10 to 360minutes.

(8) The producing method of the roughly shaped material for theinduction hardening according to (7),

wherein an average cooling rate in a temperature range of 750 to 650° C.during the cooling may be 300° C./hour or less.

(9) A producing method of a roughly shaped material for the inductionhardening,

wherein processes of hot working and cooling are performed in order byusing the steel for the induction hardening according to any one of (1)to (5), and

wherein an average cooling rate in a temperature range of 750 to 650° C.during the cooling is 300° C./hour or less.

(10) An induction hardening steel part which is produced by using thesteel for the induction hardening according to any one of (1) to (5),

wherein a hardness of a hardened surface layer at a depth of 50 μm froma topmost surface of the induction hardening steel part is HV650 ormore, a hardness of a non-induction hardening area is HV180 or more, anda number of graphite grains with an average grain size of 0.5 μm or morewhich exist in the non-induction hardening area is 40 pieces/mm² orless.

Advantageous Effects of Invention

According to the above aspects of the present invention in regard to thesteel for the induction hardening, the roughly shaped material for theinduction hardening, the producing method thereof, and the inductionhardening steel part, the fatigue strength (the tooth surface fatiguestrength, the tooth root fatigue strength, and the like) of the steelpart after the induction hardening is equal to or greater than that ofthe carburized parts, and the workability during the production of thepart is enhanced simultaneously. For the reason, it is possible tosubstitute the induction hardening treatment for the carburizingtreatment. Thus, a continuous surface hardening treatment becomespossible, a load on the environment can be reduced, and the partaccuracy can be improved. Therefore, the present invention contributesto the cost reduction, the environmental loading reduction, and theperformance improvement for the automobiles and the like through theimprovement of production method for the power transmission parts (forexample, gears, bearings, shafts, CVT sheaves, and the like) of theautomobiles and the like.

DESCRIPTION OF EMBODIMENTS

Through thorough research on various factors influencing the dispersivemorphology of carbides in a carburized layer in a high carboncarburizing treatment, and through consideration of a method to obtainthe fatigue strength which is equal to that of carburized steels for aninduction hardening steel, the inventors have acquired the followingknowledge.

(a) The 300° C. tempered hardness increases with an increase in carboncontent of a roughly shaped material for the induction hardening, andthe 300° C. tempered hardness which is equal to carburized parts isobtained by addition of C of more than 0.75%. Thus, a tooth surfacefatigue strength which is equal to that of the carburized parts can beobtained even in the parts after the induction hardening.

(b) In case that the carbon content of the steel is more than 0.75%,structure of the roughly shaped material before machining and forming ofthe parts (cutting, cold forging) becomes mostly pearlite structure.Thus, the strength (related to a pearlite lamellar spacing) of thepearlite structure has dominant influence on the hardness of the roughlyshaped material.

(c) By performing a suitable annealing in production process of theroughly shaped material before the machining and forming of the parts,fine pearlite lamellar can be changed and softening can be obtained, sothat workability can be improved.

(d) On the other hand, in case that the roughly shaped material beforethe machining and forming of the parts is made by hot working, byperforming a suitable cooling after the hot working, the pearlitelamellar spacing is broadened and softening can be obtained, so that theworkability can be improved.

(e) By combination of the above (c) and (d), the roughly shaped materialcan be further softened, so that the workability can be further improvedor an annealing time can be shortened.

(f) By securement of amount of solid soluted Al which is derived from anincrease in Al content considerably compared with a conventional steeland simultaneously suppression of N content, tool life in the cuttingcan be prolonged drastically and machinability of the roughly shapedmaterial can be improved. As to the conventional technique, the hardnessof the roughly shaped material increases with the increase in the carboncontent of the steel, so that the cutting cannot be performed. Incontrast, according to the present invention, by the securement of theamount of solid soluted Al sufficiently, the cutting can be performedeven if the hardness of the roughly shaped material increases, so thatit is possible to increase the carbon content of the steel.

(g) Cr stabilizes θ carbide (cementite) by concentrating in the θcarbide, so that it is suppressed that the carbides dissolve intoaustenite during the induction hardening, which causes hardnessunevenness of a hardening layer. Thus, in case that Cr is added, theaddition is limited. In case that V, Nb, or Ti is added, excessiveaddition also causes the hardness unevenness of the hardening layer suchas Cr, the hardness of the roughly shaped material increases, and theworkability deteriorates, so that the addition is limited.

(h) Depending on the conditions of the annealing in producing theroughly shaped material by using the steel for the induction hardening,graphite grains may be formed in the roughly shaped material. If thegraphite grains with a certain size or more and more than a certainquantity in the roughly shaped material exist when the cutting and/orcold working is conducted by using the roughly shaped material, thegraphite grains do not dissolve sufficiently into the austenite duringonly short-time heating of the induction hardening, which causes thehardness unevenness of the hardening layer. Moreover, even if thegraphite grains dissolve into the austenite, voids remain at theposition where the graphite grains existed, so that the property of theparts may deteriorate. For these reasons, it is necessary to limit anamount of precipitation of the graphite in the roughly shaped material.

Hereinafter, the present invention will be described in detail. First,reasons for limiting the chemical composition of the steel for theinduction hardening according to an aspect of the present invention willbe described. The content % of the components means % by mass.

C: more than 0.75% to 1.20%

Carbon (C) is added to secure the surface hardness after the inductionhardening and to secure the hardness of the core of the part. Ingeneral, the carbon content at the surface of the carburized parts isapproximately 0.80%. In order for the induction hardening steel part toobtain the tooth surface fatigue strength (the 300° C. temperedhardness) which is equal to that of the carburized parts, the carboncontent of the steel for the induction hardening should be increasedcompared with the conventional case. Since the tooth surface fatiguestrength which is equal to that of the carburized parts is not obtainedwhen the addition is insufficient, the carbon content of more than 0.75%needs to be added. When C of more than 1.20% is added, the hardness ofthe roughly shaped material increases, so that the workability inperforming the process such as cutting, forging, and the like of theparts deteriorates considerably. Therefore the carbon content needs tobe in the range of more than 0.75% to 1.20%. The preferable range forthe carbon content is 0.76 to 0.90%.

Si: 0.002 to 3.00%

Silicon (Si) suppresses the transition to relatively coarse the θcarbide from ε carbide which precipitates at tempering, and considerablyincreases the resistance to temper softening of the martensite steeltempered at a lower temperature, when Si is added to the high carbonsteel. Thus the tooth surface fatigue strength of the steel increases.To obtain the effect, the Si content of 0.002% or more needs to be addedto the steel for the induction hardening of the present invention.Although the effect is enhanced with an increase in the Si addition,when Si of more than 3.00% is added, the hardness of the roughly shapedmaterial increases, so that the workability in performing the processsuch as cutting, forging, and the like of the parts deterioratesconsiderably. In addition, since Si stabilizes the ferrite, when Si ofmore than 3.00% is added, the ferrite remains at the induction hardeningand the uniform austenite is not obtain. As a result, the uniformaustenite is not obtain after the hardening. Therefore the Si contentneeds to be in the range of 0.002 to 3.00%. The preferable range for theSi content is 0.20% to 1.50%. Especially, when the amount of thegraphite needs to be restricted, the Si content may be 0.50% or less.

Mn: 0.20 to 2.00%

Manganese (Mn) has an effect of improving the hardenability of steel, sothat Mn is effective in obtaining the martensite structure duringcarburizing quenching. To obtain the effect, the Mn content of 0.20% ormore needs to be added to the steel for the induction hardening of thepresent invention. On the other hand, when Mn of more than 2.00% isadded, the hardness of the roughly shaped material increases, so thatthe workability in performing the process such as cutting, forging, andthe like of the parts deteriorates considerably. Therefore the Mncontent needs to be in the range of 0.20 to 2.00%. The preferable rangefor the Mn content is 0.30% to 1.00%.

S: 0.002 to 0.100%

Sulfur (S) forms MnS by bonding to Mn. As the S addition increases, Sshows an effect of improving the machinability. To obtain the effect,the S content of 0.002% or more needs to be added to the steel for theinduction hardening of the present invention. On the other hand, when Sof more than 0.100% is added, MnS becomes a path for propagating fatiguecracks, so that the bending fatigue strength of the products such as thegear and the like decreases. Therefore the S content needs to be in therange of 0.002 to 0.100%. The preferable range for the S content is0.010% to 0.050%.

Al: more than 0.050% to 3.00%

Aluminum (Al) has an effect of improving considerably the tool life inthe cutting of the roughly shaped material in case that Al exists as asolid solution in the roughly shaped material. The effect is derivedfrom that the solid soluted Al of the roughly shaped material reactswith oxygen during the cutting, the hard coat of Al₂O₃ is formed, andthe coat suppresses wear of the tool. The solid soluted Al of theroughly shaped material reacts with oxygen in the air, oxygen in acutting oil, or oxygen in the homo treatment film (Fe₃O₄) of a toolsurface, and the coat of Al₂O₃ which protects the tool is formed. Thehomo treatment film is an iron oxidation film with a thickness ofseveral μm, which is formed by a heat treatment in a steam which is alsocalled as a steam treatment, in order for the tool to obtain a corrosionresistance and the like (reference: “heat treatment technology” whichwas written and edited by Japan Society for Heat Treatment in 2000,Nikkan Kogyo Shimbun Ltd., Tokyo, P 569). By the presence of the coatwhich protects the tool, the direct contact of a material to be cut (theroughly shaped material) and the tool is prevented, and the adhesivewear of the tool is suppressed. In conventional technique, since thewear of the tool increases considerably with the increase in thehardness of the roughly shaped material, the increase in the carboncontent of the roughly shaped material was impossible practically. Onthe other hand, in the present invention, since the increase in the wearof the tool is suppressed by adding excessive Al against the increase inthe hardness of a roughly shaped material, the industrial productionbecomes possible even if the carbon content of the steel for inductionhardening is increased compared with the conventional technique. Inaddition, Al has the same effect as Si for the tempering behavior of themartensite steel tempered at a lower temperature, and is effective inimproving the tooth surface fatigue strength by increasing considerablythe resistance to temper softening. To obtain the effect, the Al contentof 0.050% or more needs to be added to the steel for the inductionhardening of the present invention. On the other hand, Al stabilizes theferrite, when Al of more than 3.00% is added, the ferrite remains at theinduction hardening and the uniform austenite is not obtain. As aresult, the uniform austenite is not obtained after the hardening.Therefore the Al content needs to be in the range of more than 0.050% to3.00%. The preferable range for the Al content is 0.100% to 1.00%.

P: 0.050% or less

Phosphorus (P) is an unavoidable impurity, is segregated at theaustenite grain boundary, and embrittles the prior austenite grainboundary, thereby resulting in an intergranular cracking. Accordingly,it is preferable to decrease the P content as possible. Consequently, inthe present invention, the P content of the steel for the inductionhardening needs to be in a range of 0.050% or less. Although it is notnecessary to determine a lower limit of the P content for the presentinvention, an excessive cost is required in order to limit the P contentto 0.001% or less. Therefore, the preferable range for the P content is0.001% to 0.015%.

N: 0.0200% or less

Nitrogen (N) forms AlN by bonding to Al in the steel. AlN functions tosuppress the grain growth by pinning the austenite grain boundary,thereby preventing the structure from coarsening. In general, since aheating time of the induction heating is very short, grains are hard tocoarsen even if AlN is not utilized actively. However, N may be addedintentionally in order to refine the structure actively. On the otherhand, if N is excessively added, ductility in a high temperature regionof 1000° C. or higher deteriorates, which causes the decrease in yieldof continuous casting and rolling. Therefore, in the present invention,the N content of the steel for the induction hardening needs to belimited to 0.0200% or less. The preferable range for the N content is0.0050% to 0.0120%.

O: 0.0030% or less

Oxygen (O) forms oxide inclusions. If the O content is excessive, coarseinclusions which act as the origin of the fatigue fracture increase,which causes the deterioration of fatigue properties. Accordingly, it ispreferable to decrease the O content as possible. Therefore, in thepresent invention, the O content of the steel for the inductionhardening needs to be limited to 0.0030% or less. Although it is notnecessary to determine a lower limit of the O content for the presentinvention, the excessive cost is required in order to limit the Ocontent to 0.0001% or less. Therefore, the preferable range for the Ocontent is 0.0001% to 0.0015%.

B: 0.0005 to 0.0050%

Boron (B) is a selective element which can be added as necessary to thesteel for the induction hardening of the present invention. Since Bwhich is dissolved in the austenite as the solid solution has the effectof greatly improving the hardenability of the steel even in a smallamount, B is the element which is effective in obtaining the martensitestructure during the carburizing quenching. To obtain the effect, the Bcontent of 0.0005% or more may be added to the steel for the inductionhardening of the present invention. On the other hand, when the Bcontent of more than 0.0050% is added, the effect is saturated.Therefore, when B is added, the B content may be in the range of 0.0005to 0.0050%. The preferable range for the B content may be 0.0010 to0.0025%. If N of a certain amount or more exists in the steel, B bondsto N and forms BN. As a result, the effect of improving thehardenability cannot be obtained in some cases, because the amount ofsolid soluted B is reduced. When B is added, it is preferable tosimultaneously add a suitable amount of Ti and Al for fixing N.

Cr: 0.05% to less than 0.30%

Chromium (Cr) is the selective element which can be added as necessaryto the steel for the induction hardening of the present invention. SinceCr has an effect of refining considerably the lamellar spacing inpearlitic transformation, the hardness of the roughly shaped materialincreases considerably and the workability deteriorates. In addition, Crstabilizes the θ carbide by concentrating in the θ carbide, so that itis suppressed that the carbides dissolve into the austenite during theinduction hardening, which causes the hardness unevenness of thehardening layer. Therefore, when Cr is added, the Cr content may belimited to less than 0.30%. On the other hand, the 0 carbide may betransformed to the graphite in case that Si and Al content are excessiveand the annealing time is long, so that the induction-hardenability maydeteriorate. In order to prevent the situation, a small amount of Cr maybe added to the steel for the induction hardening of the presentinvention. A lower limit of Cr in order to prevent the transformation tothe graphite is 0.05%. Therefore, when Cr is added, the Cr content maybe in the range of 0.05% to less than 0.30%. The preferable range forthe Cr content may be 0.10 to 0.20%.

Mo: 0.01 to 1.00%

Molybdenum (Mo) is the selective element which can be added as necessaryto the steel for the induction hardening of the present invention. SinceMo has the effect of improving the hardenability of the steel, Mo is theelement which is effective in obtaining the martensite structure duringthe carburizing quenching. To obtain the effect, the Mo content of 0.01%or more may be added. On the other hand, when the Mo content of morethan 1.00% is added, the cost for addition rises, and the workability inperforming the process such as cutting, forging, and the like of theparts deteriorates considerably because the hardness of the roughlyshaped material increases, which is not suitable for industrialproduction. Therefore, when Mo is added, the Mo content may be in therange of 0.01 to 1.00%. The preferable range for the Mo content may be0.10 to 0.60%. In addition, in order to improve the hardenability asmuch as possible without deteriorating the workability in performing thecutting and the forging, it is preferable to add a small amount of Mo.Namely, when the range of the Mo content is 0.10 to 0.50%, thedeterioration of the workability caused by the increase in the hardnessof the roughly shaped material becomes small and negligiblesubstantially, and obvious improvement effect of the hardenability isalso obtained. The reason is that Mo is the element which has therelatively large effect to improve the hardenability even in a smalladdition. Especially, when multiple additions with B are performed, alarge effect of the multiple addition for the improvement of thehardenability is obtained even in the small addition.

Cu: 0.05 to 1.00%

Copper (Cu) is the selective element which can be added as necessary tothe steel for the induction hardening of the present invention. Since Cuhas the effect of improving the hardenability of the steel, Cu iseffective in obtaining the martensite structure during the carburizingquenching. To obtain the effect, the Cu content of 0.05% or more may beadded. However, when the Cu content of more than 1.00% is added,ductility in a high temperature region of 1000° C. or higherdeteriorates, which causes the decrease in yield of the continuouscasting and the rolling. Therefore, when Cu is added, the Cu content maybe in the range of 0.05 to 1.00%. The preferable range for the Cucontent may be 0.010 to 0.50%. In addition, in order to improve theductility of the high temperature range, it is preferable to addsimultaneously Ni whose addition is a half or more of the Cu additionwhen Cu is added.

Ni: 0.05 to 2.00%

Nickel (Ni) is the selective element which can be added as necessary tothe steel for the induction hardening of the present invention. Since Nihas the effect of improving the hardenability of the steel, Ni is theelement which is effective in obtaining the martensite structure duringthe carburizing quenching. To obtain the effect, the Ni content of 0.05%or more may be added. On the other hand, when the Ni content of morethan 2.00% is added, the cost for the addition rises, which is notsuitable for the industrial production. Therefore, when Ni is added, theNi content may be in the range of 0.05 to 2.00%. The preferable rangefor the Ni content may be 0.40 to 1.60%.

V: 0.005 to less than 0.20%

Vanadium (V) is the selective element which can be added as necessary tothe steel for the induction hardening of the present invention. V formsV(C, N) by bonding to N and C in the steel. V(C, N) functions tosuppress the grain growth by pinning the austenite grain boundary,thereby preventing the structure from coarsening. To obtain the effect,the V content of 0.005% or more may be added. On the other hand, whenthe V content of 0.20% or more is added, the hardness of the roughlyshaped material increases, so that the workability in performing theprocess such as the cutting, the forging, and the like of the partsdeteriorates considerably. In addition, formation of V(C, N) becomesexcessive, which causes the hardness unevenness of the hardening layerin the induction hardening. Therefore, when V is added, the V contentmay be in the range of 0.005 to less than 0.20%. The preferable rangefor the V content may be 0.05 to 0.10%.

Nb: 0.005 to 0.10%

Niobium (Nb) is the selective element which can be added as necessary tothe steel for the induction hardening of the present invention. Nb formsNb(C, N) by bonding to N and C in the steel. Nb(C, N) functions tosuppress the grain growth by pinning the austenite grain boundary,thereby preventing the structure from coarsening. To obtain the effect,the Nb content of 0.005% or more may be added. On the other hand, whenthe Nb content of more than 0.10% is added, the hardness of the roughlyshaped material increases, so that the workability in performing theprocess such as the cutting, the forging, and the like of the partsdeteriorates considerably. In addition, formation of Nb(C, N) becomesexcessive, which causes the hardness unevenness of the hardening layerin the induction hardening. Therefore, when Nb is added, the Nb contentmay be in the range of 0.005 to 0.10%. The preferable range for the Nbcontent may be 0.010 to 0.050%.

Ti: 0.005 to 0.10%

Titanium (Ti) is the selective element which can be added as necessaryto the steel for the induction hardening of the present invention. Tiforms Ti(C, N) by bonding to N and C in the steel. Ti(C, N) functions tosuppress the grain growth by pinning the austenite grain boundary,thereby preventing the structure from coarsening. To obtain the effect,the Ti content of 0.005% or more may be added. On the other hand, whenthe Ti content of more than 0.10% is added, the hardness of the roughlyshaped material increases, so that the workability in performing theprocess such as the cutting, the forging, and the like of the partsdeteriorates considerably. In addition, formation of Ti(C, N) becomesexcessive, which causes the hardness unevenness of the hardening layerin the induction hardening. Therefore, when Ti is added, the Ti contentmay be in the range of 0.005 to 0.50%. The preferable range for the Ticontent may be 0.015 to 0.050%.

Ca, Zr, Mg: 0.0005 to 0.0030%

Calcium (Ca), Magnesium (Mg), and Zirconium (Zr) are the selectiveelement which can be added as necessary to the steel for the inductionhardening of the present invention. Ca, Mg, and Zr respectively havefunctions to control the shape of MnS and to improve the machinabilityof the steel by forming a protective coat for the surface of the cuttingtool during the cutting. To obtain the effect, at least one of Ca, Mg,and Zr of 0.0005% or more may be added. On the other hand, when theaddition is more than 0.0030%, coarse oxides and sulfides may be formed,so that the fatigue strength of the part may be negatively influenced insome cases. Therefore, when Ca, Mg, or Zr is added, the Ca, Mg, or Zrcontent may be in the range of 0.0005 to 0.0030%. The preferable rangefor the Ca, Mg, and Zr content may be 0.0008 to 0.0020%.

In the present invention, Lead (Pb), Tellurium (Te), Zinc (Zn), Tin(Sn), and the like can be added in a range that does not diminish theeffects of the present invention, in addition to the above elements. Pb,Te, Zn, and Sn are the selective element which can be added as necessaryto the steel for the induction hardening of the present invention. Inorder not to diminish the effect of the present invention, an upperlimit of an amount of these elements may be Pb: 0.50% or less, Te:0.0030% or less, Zn: 0.50% or less, and Sn: 0.50% or less, respectively.Al−(27/14)×N>0.050%

As mentioned above, Al has the effect of improving considerably the toollife in the cutting of the roughly shaped material in case that Alexists as a solid solution in the steel, so that the addition of Al isin the range of more than 0.050% to 3.00%. On the other hand, since Almay form AlN by bonding to N in the steel, Al may take morphology of theprecipitates. However, Al which exists as the precipitates is noteffective in improving the tool life. Especially, like the presentinvention, when performing slow cooling after hot forging or performingthe annealing before the cutting, AlN precipitates easily compared withthe process which performs air cooling after the hot forging. Therefore,in order to secure the solid soluted Al sufficiently, it is necessary toadd Al more excessively than the predictive quantity to form AlN, sothat it is needed to provide the relational expression between Al and N.Namely, when the value of “Al−(27/14)×N” which is an equation of theparameter of the solid soluted Al quantity is more than 0.050%, theimprovement effect of the tool life can be obtained certainly. In regardto the steel for the induction hardening of the present invention, atheoretical upper limit of “Al−(27/14)×N” is 3.00%, and a preferablerange is 0.100 to 1.00%.

In the roughly shaped material for the induction hardening according toan aspect of the present invention, coexistence between the toothsurface fatigue strength and the workability is attained sufficiently bycontrolling the steel composition and annealing conditions. Moreover,the formation of the coarse graphite grains is to be suppressed, and thenumber of the graphite grains with an average grain size of 0.5 μm ormore is to be 40 pieces/mm² or less. When the amount of the graphitegrains in the roughly shaped material is in the range, since thehardness of the hardening layer becomes even after performing theinduction hardening to the roughly shaped material, it is possible tosuppress the formation of the voids which are derived from the graphitegrains. If the annealing with the suitable conditions is performed whenproducing the roughly shaped material by using the steel for theinduction hardening of the present invention, it is possible that thenumber of the graphite grains with the average grain size of 0.5 μm ormore is to be 0 pieces/mm². In other words, since the graphite may formin some steel compositions when performing excessive slow cooling aftercasting or performing the annealing for 300 min or more in a temperaturerange of 600° C. to 720° C. as annealing temperature, the formation ofthe graphite can be suppressed by avoiding the annealing for excessivelong time in the above temperature range.

When performing the annealing at high temperature and for long time, itis preferable that graphitization value CE, which is defined as thefollowing equation (1), is controlled to 1.8 or less by controlling thecompositions of the steel for the induction hardening. Especially whenperforming the annealing at high temperature, it is more preferable thatthe CE is controlled to 1.28 or less.CE=C+Si/3−Mn/12+Al/6+Cu/9+Ni/9−Cr/9−Mo/9+B  (1)

where the C, Si, Mn, Al, Cu, Ni, Cr, Mo, and B in the equation (1)indicate mass % of each element which is included in the steel for theinduction hardening.

In a producing method of the roughly shaped material for the inductionhardening according to an aspect of the present invention, processes ofwarm working or hot working, cooling, and annealing are performed inorder, by using the steel for the induction hardening which has theabove-mentioned composition. The annealing is performed by a conditionof an annealing temperature of 680 to 800° C. and an annealing time of10 to 360 minutes. The reason for using the conditions is explainedbelow.

An instance of the warm working is warm forging, and an instance of thehot working is hot forging or hot rolling. When producing the roughlyshaped material by performing the warm working or the hot working tocomparative low carbon steels according to the conventional technique,the structure of the roughly shaped material becomes mainly (95% ormore) the ferrite or the pearlite. In this case, the hardness of theroughly shaped material is greatly affected by the amount of a softferrite or the hardness of the ferrite itself. In order to soften theroughly shaped material, there are a method of increasing the fractionof the ferrite by combining the working and heat treating, a method ofsuppressing the addition of the element which has an effect of solutestrengthening to the ferrite, and the like.

On the other hand, in the steel for the induction hardening of thepresent invention, the carbon content exceeds 0.75%. For this reason,even if the roughly shaped material is produced by any of the warmforging, the hot forging, or the hot rolling by using the steel, thestructure of the roughly shaped material becomes the structure in whichlarge fraction is the pearlite and small fraction is the ferrite, or thestructure in which whole (95% or more) is substantially the pearlite.Therefore, the strength of the pearlite structure has dominant influenceon the strength of the roughly shaped material. The strength of thepearlite structure is related to the lamellar spacing of the pearlite.In order to soften the steel which includes mainly the pearlite, it isexceedingly effective to obtain the structure in which the θ carbidesdisperse roughly, by changing the morphology of fine pearlite lamellarthrough the annealing. Namely, the effect of softening by the annealingin the case of the pearlite structure of the high carbon steel is largerthan that in the case of the ferrite and pearlite structure of the lowand medium carbon steel. In addition, when the heating temperature ofthe annealing is low, sufficient softening effect is not obtainedbecause the ferrite and pearlite structure hardly change. Thus, it isnecessary to perform the annealing at the temperature of 680° C. ormore. In general, with an increase in the heating temperature, the finepearlite lamellar is transformed and the θ carbides disperse roughly.However, when the annealing temperature is more than 800° C., theformation of the austenite increases and the austenite is transformed tothe pearlite with the fine lamellar again during the cooling from theannealing temperature, so that the softening effect is not obtained.Therefore, the annealing temperature needs to be in the range of 680 to800° C. The preferable range for the annealing temperature is 700 to770° C. When the annealing time is short, since the morphology of thepearlite lamellar hardly changes, the sufficient softening effect is notobtained. Therefore the heating of the annealing needs to be 10 minutesor more. On the other hand, when the heating of the annealing is morethan 360 minutes, the productivity decreases, which is not suitable forindustrial production. Therefore, the heating time needs to be in therange of 10 to 360 minutes. The preferable range for the heating time is30 to 300 minutes. In addition, although the cooling condition after theannealing is not provided in particular, it is preferable to perform theslow cooling as necessary, because the steel further softens by thecooling with small cooling rate (the slow cooling). The preferable rangefor an average cooling rate in a temperature range of 750 to 650° C.during the cooling is 300° C./hour or less.

In a producing method of the roughly shaped material for the inductionhardening according to another aspect of the present invention,processes of hot working and cooling are performed in order, by usingthe steel for the induction hardening which has the above-mentionedcomposition. In the cooling process which follows the hot workingprocess, an average cooling rate in a temperature range of 750 to 650°C. is 300° C./hour or less. In this aspect, an annealing does notnecessarily perform. The reason for using this cooling condition isexplained below.

As mentioned above, the strength of the pearlite structure has dominantinfluence (the pearlite lamellar spacing) on the hardness of the roughlyshaped material, so that the annealing is exceedingly effective insoftening. However, in order to improve the cost and the productivityfor producing the parts, it is advantageous to omit the annealing. Forthis reason, the cooling rate which follows the hot working such as thehot forging or the hot rolling is controlled, the slow cooling isconducted in a temperature range of the pearlitic transformation, andthe pearlite lamellar spacing is broadened by the pearlitictransformation at high temperature. As a result, the structure of thesteel can be softened. In addition, since the steel stays continuouslyin the high temperature range after completion of the pearlitictransformation by performing the slow cooling, the same effect as theannealing can also be obtained. When the temperature range for the slowcooling is more than 750° C., the softening effect is not obtained,because the slow cooling is conducted in a temperature range where thepearlitic transformation cannot occur. On the other hand, when thetemperature range for the slow cooling is less than 650° C., thepearlitic transformation begins at low temperature. Thus, broadening ofthe pearlite lamellar spacing becomes insufficient, softening alsobecomes insufficient, and moreover the annealing effect after thepearlitic transformation by the slow cooling decreases. Therefore, thetemperature range for the slow cooling needs to be in the range of 750to 650° C. The preferable range for the temperature range for the slowcooling is 740 to 680° C. In addition, when the average cooling rate ismore than 300° C./hour, since the time for the stay in the hightemperature range just after the completion of the pearlitictransformation is insufficient, the annealing effect decreases.Therefore, the average cooling rate in the temperature range for theslow cooling needs to be 300° C./hour or less. The preferable range forthe average cooling rate in the temperature range for the slow coolingis 200° C./hour or less. The cooling rate as limited above is theaverage cooling rate from 750° C. to 650° C. and continuous cooling isnot necessarily required, so that the cooling may have a holding periodat a constant temperature in the cooling process as long as the aboveconditions are satisfied. In order to secure the productivity, it ispreferable that a lower limit of the average cooling rate is 80° C./houror more.

When performing the slow cooling as mentioned above, it is not necessaryto perform the annealing after the cooling. However, it may be possibleto combine the slow cooling with the annealing of the above mentionedconditions. In this case, large softening effect is obtained as comparedwith performing independently the slow cooling and the annealing.

An induction hardening steel part according to an aspect of the presentinvention is produced by performing the cutting and/or the cold workingand the induction hardening, and performing furthermore low temperaturetempering as necessary, by using the roughly shaped material for theinduction hardening which is produced by any of the above mentionedproducing method. The steel part is produced in order that a hardness ofa hardened surface layer at a depth of 50 μm from a topmost surface ofthe induction hardening steel part is HV 650 or more and a hardness of anon-induction hardening area is HV 180 or more. The reason for thelimitations is explained below. Instances of the steel part are the mostpower transmission parts and the like (for example, gears, bearings, CVTsheaves, and shafts) used for automobiles, construction machines, farmmachines, electricity generating-wind turbines, other industrialmachines and the like.

In order to obtain the fatigue properties and wear properties, surfacehardening is performed to the parts such as the CVT sheaves, bearings,and the like. In the steel part according to the present invention, theinduction hardening treatment corresponds to the surface hardening. Inorder to secure the fatigue properties and the wear properties which areequal to those of the carburized parts by this treatment, it isnecessary to increase the surface hardness to the hardness which isequal to that of the carburized parts. The hardness at the depth of 50μm from the topmost surface was selected as a representative value ofthe surface hardness. When the hardness at the position is HV 650 ormore, it can be judged as the hardness which is equal to that of thegeneral carburized parts. In this case, the fatigue properties and thewear properties which are equal to those of the carburized parts areobtained. A preferable upper limit of the hardness of the inductionhardening area of the part which is produced by the steel compositionand the producing method according to the present invention is HV 950level. The preferable range for the hardness of the above area is HV 700or more.

The details of processing conditions of the induction hardening(electromagnetic induction hardening) change with shape of the part andthe like, and general techniques which are publicly known can beutilized. As an instance of the induction hardening which fits thepresent invention, the following condition can be used: for example, thequenching by the electromagnetic induction is performed on the frequencyof 10 to 500 kHz and the processing time of 0.1 to 20 seconds by using aring coil; subsequently the quenching by water cooling is performed; anda hardening depth becomes 0.2 to 2.5 mm. When performing the heating bythe electromagnetic induction, the processed parts may be rotated at 100to 2000 rpm in order to homogenize the depth of the hardening layer andto perform contour hardening of the gears. Moreover, in order to performthe rapid and short time heating, preheating may be conducted to atemperature range of Al point or less by low frequency electromagneticinduction beforehand.

Depending on the processing conditions, the hardening by the inductionhardening can reaches from the surface of the induction hardening steelpart to a depth range of 0.1 mm to 3 mm approximately, and themeaningful hardening does not occur at the depth more than above, thatis inside of the steel (core area). The non-hardening area as mentionedabove is referred to as the non-induction hardening area. Therefore, thehardness of the non-induction hardening area is substantially equal tothe hardness of the roughly shaped material before the inductionhardening. Since the hardness of the non-induction hardening area isrelated to the fatigue strength of an interior origin and the low cyclefatigue strength of the gears, excessive low value is not suitable. Onthe other hand, since the strength of whole of the induction hardeningsteel part can be improved by controlling the depth of the inductionhardening, the interior hardness may be low to a certain extent comparedwith that of the general carburized parts. Especially, in order tosecure the low cycle fatigue strength, the hardness of the non-inductionhardening area needs to be HV 180 or more is needed, and the preferablerange is HV 200 or more. Since the roughly shaped material according tothe present invention can maintain the sufficient workability by theeffect of the solid soluted Al even if the hardness is high, it ispossible to secure sufficiently the hardness of the non-inductionhardening area. In order to secure the workability before the hardening,a preferable upper limit of the hardness of the non-induction hardeningarea of the steel part according to the present invention is HV 240.

Shot peening may be performed after the induction hardening or after theinduction hardening and the low temperature tempering (300° C. or less)to the induction hardening steel part according to the above aspect ofthe present invention. Since an increase in the compressive residualstress of the surface layer of the part, which is induced by the shotpeening process, results in the suppression of initiation andpropagation of fatigue crack, the tooth root and the tooth surfacefatigue strength of the part which is produced by the steel of thepresent invention can be further improved. It is preferable that theshot peening process is conducted by the conditions which are that shotparticles with a diameter of 0.7 mm or less are used and arc height is0.4 mm or more.

EXAMPLE

Hereinafter, examples of the present invention will be described.

Molten steel made by a converter having the composition shown in Table 1was subjected to a continuous casting, a soaking diffusion treatment asnecessary, and then a blooming rolling, thereby a rolling material of162 mm square was produced. Thereafter, the steel for the inductionhardening whose shape is a bar with diameter of 45 mm was produced bythe hot rolling.

TABLE 1 steel chemical composition (mass %) No. C Si Mn P S Cr Mo Ni CuV Ti Nb A 0.80 0.25 0.40 0.009 0.016 0.11 0.04 — — — — — B 0.80 1.000.40 0.005 0.015 — — — — — — — C 0.80 1.00 0.40 0.010 0.015 0.10 — — — —— — D 0.80 1.02 0.40 0.011 0.002 0.09 — — — — — — E 0.76 0.20 0.25 0.0130.047 — 1.00 — — — — — F 1.20 0.05 0.40 0.010 0.015 — — — — — 0.005 — G0.76 0.002 2.00 0.010 0.019 0.05 — — — — — — H 0.76 3.00 0.38 0.0230.052 0.20 — — —  0.005 — — I 0.77 0.30 0.22 0.010 0.015 — — 2.00 1.00 —— 0.018 J 0.80 0.25 0.60 0.010 0.007 0.30 — — — — 0.100 — K 0.80 0.290.39 0.012 0.015 0.10 0.11 — — 0.19 — — L 0.80 0.35 0.80 0.002 0.0150.12 — 0.08 — — — 0.100 M 0.80 0.24 0.40 0.010 0.015 0.11 — 0.05 0.05 —0.035 — N 0.53 0.25 0.76 0.015 0.019 0.10 — — — — — — O 0.60 1.63 0.410.016 0.016 0.15 — — — — — — P 0.80 0.25 0.34 0.010 0.015 0.50 — — — — —— Q 0.80 0.25 0.40 0.010 0.015 — — — — 0.33 — — R 0.80 0.26 0.43 0.0110.013 0.11 — — — — — — S 0.80 0.20 0.40 0.005 0.023 0.08 — — — — — — T0.76 0.25 0.48 0.015 0.018 — — — — — — — U 1.31 0.25 0.41 0.010 0.0250.10 — — — — — — V 1.03 2.78 0.21 0.046 0.013 — — — — — — — chemicalcomposition (mass %) steel Al − No. Al B Ca, Zr, Mg N O CE (27/14) × Nnote A 0.104 0.0020 — 0.0085 0.0016 0.85 0.088 example B 0.131 — —0.0074 0.0018 1.12 0.117 C 1.000 — — 0.0045 0.0008 1.26 0.991 D 1.400 —— 0.0040 0.0010 1.33 1.392 E 0.111 — — 0.0075 0.0011 0.71 0.097 F 0.108— Mg: 0.0010 0.0081 0.0030 1.20 0.092 G 3.000 — — 0.0035 0.0022 1.092.993 H 0.110 — Zr: 0.0012 0.0083 0.0001 1.72 0.094 I 0.120 — — 0.01560.0015 1.21 0.090 J 0.051 — — 0.0004 6.0023 0.81 0.050 K 0.121 — —0.0081 0.0019 0.86 0.105 L 0.101 — Ca: 0.0010 0.0019 0.0026 0.86 0.097 M0.110 0.0015 — 0.0033 0.0010 0.87 0.104 N 0.035 — — 0.0056 0.0016 0.550.024 comparative O 0.131 — — 0.0052 0.0013 1.11 0.121 example P 0.147 —— 0.0031 0.0018 0.82 0.141 Q 0.122 — — 0.0062 0.0008 0.87 0.110 R 0.031— — 0.0060 0.0011 0.84 0.019 S 0.025 — — 0.0050 0.0010 0.83 0.015 T0.021 — Ca: 0.0011 0.0032 0.0008 0.81 0.015 U 0.275 — Ca: 0.0012 0.00480.0021 1.39 0.266 V 0.087 — — 0.0145 0.0015 1.95 0.059

In Table 1, “-” of each element means that the element was not added.The underlined value of comparative example means out of the range ofthe present invention.

Next, in order to simulate the gear-manufacturing process (thermalhistory), the hot working or the warm working were simulated under acondition shown in Table 2 for the hot rolled steel (the steel for theinduction hardening). Heating temperature for hot working simulating was1250° C., and heating temperature for warm working simulating was 750°C. After the hot or the warm working simulating, the annealing wasconducted as necessary under a condition shown in Table 2. From samplesof the roughly shaped material produced by the above processes,machinability evaluation specimens whose shape was a disc with 45 φ×15mm and roller pitting test specimens which had a large diameter part(test part) of 26 φ were prepared.

Among the disc specimens of each test level, Vickers hardness at aposition of ¼ of the diameter in a cross section along the diameterdirection was measured for respective one specimen. When the hardness ofthe roughly shaped material was more than HV 240, it was judged asunsatisfied workability (cold forgeability or machinability).

TABLE 2 cooling rate after number of graphite hot working or hardnessgrains with average production steel working warm working annealing ofbase grain size of 0.5 μm No. No. process ° C./h condition steel HV ormore pieces/mm² 1 A hot working uncontrolled (air cooling) 720° C. × 300min. 190 0 2 A hot working 130 none 205 0 3 A hot working 200 720° C. ×300 min. 180 0 4 A hot working 300 720° C. × 30 min.  190 0 5 A warmworking uncontrolled (air cooling) 720° C. × 30 min.  185 0 6 B hotworking uncontrolled (air cooling) 740° C. × 300 min. 200 0 7 B hotworking 130 none 215 0 8 B hot working 200 740° C. × 300 min. 190 0 9 Bhot working 300 740° C. × 30 min.  200 0 10 B warm working 300 740° C. ×300 min. 190 0 11 C hot working uncontrolled (air cooling) 760° C. × 300min. 200 0 12 C hot working 130 none 215 0 13 C hot working 200 760° C.× 300 min. 190 0 14 C hot working 300 760° C. × 30 min.  200 0 15 C warmworking uncontrolled (air cooling) 760° C. × 300 min. 185 0 16 D hotworking uncontrolled (air cooling) 765° C. × 300 min. 200 0 17 E hotworking 100 720° C. × 300 min. 220 0 18 F hot working 100 720° C. × 360min. 230 5 19 G hot working uncontrolled (air cooling) 800° C. × 300min. 180 0 20 H hot working uncontrolled (air cooling) 780° C. × 360min. 200 13  21 I hot working 100 720° C. × 300 min. 220 0 22 J hotworking uncontrolled (air cooling) 680° C. × 300 min. 225 0 23 K hotworking uncontrolled (air cooling) 720° C. × 300 min. 240 0 24 L hotworking uncontrolled (air cooling) 720° C. × 300 min. 220 0 25 M hotworking uncontrolled (air cooling) 720° C. × 300 min. 190 0 26 A hotworking uncontrolled (air cooling) none 300 0 27 A hot workinguncontrolled (air cooling) 650° C. × 300 min. 270 0 28 A hot workinguncontrolled (air cooling) 820° C. × 300 min. 290 0 29 A hot working 350660° C. × 300 min. 260 0 30 N hot working uncontrolled (air cooling)720° C. × 300 min. 145 0 31 O hot working uncontrolled (air cooling)720° C. × 300 min. 160 0 32 P hot working uncontrolled (air cooling)720° C. × 300 min. 240 0 33 Q hot working uncontrolled (air cooling)720° C. × 300 min. 250 0 34 R hot working uncontrolled (air cooling)720° C. × 300 min. 190 0 35 S hot working 100 720° C. × 300 min. 180 038 T hot working 100 720° C. × 300 min. 175 0 37 U hot workinguncontrolled (air cooling) 720° C. × 300 min. 270 0 38 V hot workinguncontrolled (air cooling) none 235 442  39 A hot working uncontrolled(air cooling) 720° C. × 600 min. 145 155  300° C. tool life temperedhardness of roller pitting in cutting hardness of hardness ofnon-induction fatigue production of base hardening hardening hardeningstrength No. steel m/min layer HV layer HV area HV MPa note 1 125 790690 190 3400 example 2 110 780 690 205 3400 3 135 775 690 180 3400 4 125775 690 190 3400 5 120 785 690 185 3400 6 110 810 710 200 3700 7 115 800710 215 3700 8 125 795 710 190 3700 9 110 800 710 200 3700 10 125 805710 190 3700 11 110 805 720 200 3900 12 115 815 720 215 3900 13 125 815720 190 3900 14 110 810 720 200 3900 15 120 815 720 185 3900 16 110 820725 200 4000 17  95 800 700 220 3600 18  80 845 750 230 4300 19 130 795705 180 3600 20 110 825 730 200 4000 21  90 785 685 220 3300 22  80 785895 225 3500 23  70 775 895 240 3500 24  90 785 695 220 3500 25 120 775695 190 3500 26  5 790 690 300 3400 comparative 27  40 790 690 270 3400example 28  25 790 690 290 3400 29  35 790 690 260 3400 30 135 645 610145 2200 31 130 675 620 160 2400 32  60 690 600 240 2100 33  50 715 620250 2300 34  55 785 695 190 3500 35  60 775 690 180 3400 38  65 740 660175 2700 37  40 750 720 270 3900 38  75 630 590 235 1900 39 110 620 580145 1800

“Cooling rate after hot working or warm working” in Table 2 shows theaverage cooling rate in a temperature range of 750 to 650° C. Theunderline in steel No., cooling rate, annealing condition, hardness ofhardening layer, and hardness of non-induction hardening area in Table 2means out of the range of the present invention. The underline inhardness of base steel, tool life in cutting of base steel, 300° C.tempered hardness of hardening layer, and roller pitting fatiguestrength means unsatisfied value.

The machinability evaluation test (measurement of the tool life) wasconducted under a condition shown in Table 3 by using the above the discspecimens. The maximum cutting speed (m/min) which was obtained bydrilling to the total depth of 1000 mm was used as the parameter of thetool life in the condition of the machinability evaluation test. Whenthe parameter did not reach 70 m/min, it was judged as unsatisfiedmachinability.

TABLE 3 cutting condition drill cutting speed 10~140 m/min φ3 mm feedrate 0.25 mm/rev NACHI(trademark)heiss drill water-soluble cutting oilwas utilized. (drill of type SD3.0 of nachi- fujikoshi corporation)

The induction hardening was conducted under a condition such that thedepth of the hardening layer became 2 mm at the large diameter part(test part), by using the above roller pitting test specimens.Subsequently, the tempering was conducted under a condition of 150°C.×90 minutes. Thereafter, in order to increase the accuracy of thefatigue test, grip section was subjected to a finish machining. Theroller pitting test was performed under a condition of a big roller: SCM420 carburized product-crowning 150R, frequency of rotation: 2000 rpm,lubricant oil: transmission oil, oil temperature of 80° C., slip ratioof 40%, and the maximum number of the test of ten million times, and thefatigue limit was determined by creating an S-N diagram, whereby theroller pitting fatigue strength was determined. When the roller pittingfatigue strength did not reach 2600 MPa, it was judged as unsatisfiedtooth surface fatigue strength.

One specimen of each test level in the roller pitting test specimenswhich were subjected to the induction hardening and the tempering wascut at the large diameter part for respective production No., and theVickers hardness at a position of 50 μm from the surface in the crosssection was measured. The measurement result is used as the hardness ofhardening layer. The tempering was further conducted under a conditionof 300° C.×90 minutes by using another specimen for respectiveproduction No., the respective specimen was cut at the large diameterpart, and the 300° C. tempered hardness was determined by measuring theVickers hardness at the position of 50 μm from the surface in the crosssection. When the 300° C. tempered hardness did not reach HV 630, it wasjudged as unsatisfied tooth surface fatigue strength because the 300° C.tempered hardness is inferior.

The evaluated results were shown in Table 2. All of the examples ofproduction No. 1 to 25 satisfied the target, and showed the excellentworkability and the sufficient tooth surface fatigue strength. On theother hand, in production No. 26, although the steel composition waswithin the range of the present invention, since the slow cooling andthe annealing after the hot working were not performing, the hardness ofthe roughly shaped material was high, so that the workability wasinferior. Since the annealing temperature was excessively low inproduction No. 27 and the annealing temperature was excessively high inproduction No. 28, the hardness of the roughly shaped material was high,so that the workability was inferior. In production No. 29, since thecooling rate after the hot working was excessively fast and theannealing temperature was excessively low, the hardness of the roughlyshaped material was high, so that the workability was inferior. Inproduction No. 30 and 31, although the hardness of the roughly shapedmaterial was low and the workability was excellent because the carboncontent was low, the 300° C. tempered hardness was low and the rollerpitching fatigue strength was also low. In production No. 32, since theCr content was excessive, the softening effect by the slow cooling orthe annealing was not obtained sufficiently, so that the workability wasinferior. In addition, since it was not sufficient that the carbidesdissolved into the austenite during the induction hardening, thehardness of the hardening layer was not obtained sufficiently, so thatthe 300° C. tempered hardness became low and the roller pitching fatiguestrength was also low. In production No. 33, since the V content wasexcessive, the softening effect by the slow cooling or the annealing wasnot obtained sufficiently, so that the workability was inferior. Inaddition, since it was not sufficient that the carbides dissolved intothe austenite during the induction hardening, the hardness of thehardening layer was not obtained sufficiently, so that the 300° C.tempered hardness became low and the roller pitching fatigue strengthwas also low. In production No. 34, 35, and 36, since the Al wasinsufficient, the improvement effect of the tool life by the solidsoluted Al was not obtained, so that the tool life in the cutting wasinferior even in low value of the hardness of the roughly shapedmaterial. In production No. 37, since the carbon content was excessive,the softening to the target value was not obtained even if the slowcooling and an annealing were conducted, so that the workability wasinferior. In production No. 38 which was steel No. V, although each ofthe steel composition was within the range of the present invention, theCE value was out of the preferable value of the present invention, sothat the graphite of more than the range of the present inventionprecipitated in the roughly shaped material. Thereby, the hardness ofthe hardening layer after the induction hardening and the 300° C.hardness of the hardening layer were insufficient. In addition, in thehardening layer, the roller pitching fatigue strength was also lowbecause the voids were formed at the position where the graphitesexisted. In production No. 39, although the steel composition was withinthe range of the present invention, since the annealing time forproducing the roughly shaped material was excessively long, the graphiteof more than the range of the present invention precipitated. Thereby,the hardness of the hardening layer after the induction hardening andthe 300° C. hardness of the hardening layer were insufficient. Moreover,the hardness of the non-induction hardening area after the inductionhardening was also necessarily low because the hardness of the roughlyshaped material decreased by precipitation of the graphite, and theroller pitching fatigue strength was also low because the voids existedin the hardening layer.

INDUSTRIAL APPLICABILITY

The steel for the induction hardening, the roughly shaped material forthe induction hardening, the producing method thereof, and the inductionhardening steel part according to the above aspects of the presentinvention can apply to most power transmission parts (for example,gears, bearings, CVT sheaves, and shafts) used for automobiles,construction machines, farm machines, electricity generating-windturbines, other industrial machines and the like. In addition, thecoexistence between the workability during the production of the partand the fatigue strength of the steel part after the induction hardeningcan be achieved. Thus it is possible to substitute the inductionhardening treatment for the carburizing treatment. Therefore thecontinuous surface hardening treatment becomes possible, the load on theenvironment can be reduced, and the part accuracy can be improved.

The invention claimed is:
 1. A roughly shaped material for an inductionhardening comprising, by mass %, C: more than 0.75% to 1.20%, Si: 0.002to 3.00%, Mn: 0.20 to 2.00%, S: 0.002 to 0.100%, Al: more than 0.050% to3.00%, P: limited to 0.050% or less, N: limited to 0.0200% or less, O:limited to 0.0030% or less, at least one of V: 0.005 to less than 0.20%,Nb: 0.005 to 0.10%, and Ti: 0.005 to 0.10%, and the balance of iron andunavoidable impurities, wherein an Al content and a N content, by mass%, satisfy Al−(27/14)×N>0.050%, and wherein the roughly shaped materialhas a number of graphite grains with an average grain size of 0.5 μm ormore, which is included in the roughly shaped material for inductionhardening is 40 pieces/mm² or less.
 2. The roughly shaped material forinduction hardening according to claim 1, further comprising, by mass %,B: 0.0005 to 0.0050%.
 3. The roughly shaped material for inductionhardening according to claim 1, further comprising at least one of, bymass %, Cr: 0.05% to less than 0.30%, Mo: 0.01 to 1.00%, Cu: 0.05 to1.00%, and Ni: 0.05 to 2.00%.
 4. The roughly shaped material forinduction hardening according to claim 1, further comprising at leastone of, by mass %, Ca: 0.0005 to 0.0030%, Zr: 0.0005 to 0.0030, and Mg:0.0005 to 0.0030%.
 5. The roughly shaped material for inductionhardening according to claim 2, further comprising at least one of, bymass %, Cr: 0.05% to less than 0.30%, Mo: 0.01 to 1.00%, Cu: 0.05 to1.00%, and Ni: 0.05 to 2.00%.
 6. The roughly shaped material forinduction hardening according to claim 2, further comprising at leastone of, by mass %, Ca: 0.0005 to 0.0030%, Zr: 0.0005 to 0.0030%, and Mg:0.0005 to 0.0030%.
 7. The roughly shaped material for inductionhardening according to claim 3, further comprising at least one of, bymass %, Ca: 0.0005 to 0.0030%, Zr: 0.0005 to 0.0030%, and Mg: 0.0005 to0.0030%.
 8. The roughly shaped material for induction hardeningaccording to claim 5, further comprising at least one of, by mass %, Ca:0.0005 to 0.0030%, Zr: 0.0005 to 0.0030%, and Mg: 0.0005 to 0.0030%. 9.A producing method of a roughly shaped material for induction hardening,the method comprising the following successive steps: warm working orhot working, cooling, and annealing a steel for induction hardeninghaving a chemical composition of the roughly shaped material forinduction hardening according to any one of claims 1-3, 4, 5 and 6-8,wherein the annealing is performed at an annealing temperature of 680 to800° C. and an annealing time of 10 to 360 minutes, and wherein anaverage cooling rate in a temperature range of 750 to 650° C. during thecooling is 300° C./hour or less.
 10. A producing method of a roughlyshaped material for induction hardening, the method comprising thefollowing successive steps: hot working and cooling a steel forinduction hardening having a chemical composition of the roughly shapedmaterial for the induction hardening according to any one of claims 1-3,4, 5 and 6-8, wherein an average cooling rate in a temperature range of750 to 650° C. during the cooling is 300° C./hour or less.
 11. Aninduction hardening steel part which is produced using the roughly shapematerial according to any one of claims 1-3, 4, 5 and 6-8, wherein ahardness of a hardened surface layer at a depth of 50 nm from a topmostsurface of the induction hardening steel part is HV650 or more, whereina hardness of a non-induction hardening area is HV180 or more, andwherein a number of graphite grains with an average grain size of 0.5 μmor more which exist in the non-induction hardening area is 40 pieces/mm²or less.